We have recently reported the characterization of two major DNA polymerases in nuclei of cells from acute myelogenous leukemia. The large enzyme (M.W. ca. 160,000) is sulfhydryl dependent and requires all four nucleoside triphosphates for activity using activated DNA as template. The smaller enzyme (M.W. ca 40,000) is not sensitive to sulfhydryl inhibitors, can incorporate single nucleoside triphosphates and has different pH and Mg ions optima than the heavy enzyme. The heavy enzyme has not been found in leukemia nuclei by previous investigators.